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Satellite picture of the Atafu atoll in Tokelau in the Pacific Ocean Atafutrim.jpg
Satellite picture of the Atafu atoll in Tokelau in the Pacific Ocean

An atoll ( /ˈætɒl,ˈætɔːl,ˈætl,əˈtɒl,əˈtɔːl,əˈtl/ ), [1] [2] sometimes known as a coral atoll, is a ring-shaped coral reef, including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim. [3] (p60) [4]


The typical atoll was originally formed as an oceanic volcano. A coral reef grew around the shore of the volcano and then, over several million years, the volcano went extinct and eroded and subsided completely beneath the surface of the ocean. The reef and the small coral islets on top of it are all that is left of the original island, and a lagoon has taken the place of the former volcano. For the atoll to persist, the coral reef must be maintained at the sea surface, with coral growth matching any relative change in sea level (subsidence of the island or rising oceans). [5]


The word atoll comes from the Dhivehi (an Indo-Aryan language spoken on the Maldive Islands) word atholhu (Dhivehi: އަތޮޅު, [ˈət̪ɔɭu] ), meaning the palm (of the hand). OED Its first recorded use in English was in 1625 as atollon. Charles Darwin recognized its indigenous origin and coined, the definition of atolls as "circular groups of coral islets", which is synonymous with "lagoon-island", in his The Structure and Distribution of Coral Reefs . [6] (p2)

More modern definitions of atoll describe them as "annular reefs enclosing a lagoon in which there are no promontories other than reefs and islets composed of reef detritus" [7] or "in an exclusively morphological sense, [as] a ring-shaped ribbon reef enclosing a lagoon". [8]

Distribution and size

Penrhyn atoll Penrhyn Aerial EFS 1280.jpg
Penrhyn atoll
NASA satellite image of some of the atolls of the Maldives, which consists of 1,322 islands arranged into 26 atolls Maldives.visibleearth.nasa.jpg
NASA satellite image of some of the atolls of the Maldives, which consists of 1,322 islands arranged into 26 atolls
Nukuoro from space. Courtesy NASA. Nukuoro ISS013-E-28610.jpg
Nukuoro from space. Courtesy NASA.
Los Roques Archipelago in Venezuela, the largest marine national park in Latin America, from space. Courtesy NASA. Los Roques.png
Los Roques Archipelago in Venezuela, the largest marine national park in Latin America, from space. Courtesy NASA.
View of the coast of Bikini Atoll from above Bikini Atoll Nuclear Test Site-115017.jpg
View of the coast of Bikini Atoll from above
Raa Atoll in Maldives Maamigili Island Raa Atoll.jpg
Raa Atoll in Maldives
Kaafu Atoll in Maldives Guriadhoo-2019-aerial-view-Luka-Peternel.jpg
Kaafu Atoll in Maldives

There are approximately 440 atolls. [10]

Most of the world's atolls are in the Pacific Ocean (with concentrations in the Tuamotu Islands, Caroline Islands, Marshall Islands, Coral Sea Islands, and the island groups of Kiribati, Tuvalu and Tokelau) and Indian Ocean (the Atolls of the Maldives, the Lakshadweep Islands, the Chagos Archipelago, and the Outer Islands of the Seychelles). The Atlantic Ocean has no large groups of atolls, other than eight atolls east of Nicaragua that belong to the Colombian department of San Andres and Providencia in the Caribbean.

Reef-building corals will thrive only in warm tropical and subtropical waters of oceans and seas, and therefore atolls are found only in the tropics and subtropics. The northernmost atoll of the world is Kure Atoll at 28°24′ N, along with other atolls of the Northwestern Hawaiian Islands. The southernmost atolls of the world are Elizabeth Reef at 29°58′ S, and nearby Middleton Reef at 29°29′ S, in the Tasman Sea, both of which are part of the Coral Sea Islands Territory. The next southerly atoll is Ducie Island in the Pitcairn Islands Group, at 24°40′ S.

Bermuda is sometimes claimed as the "northernmost atoll" at a latitude of 32°24′ N. At this latitude, coral reefs would not develop without the warming waters of the Gulf Stream. However, Bermuda is termed a pseudo-atoll because its general form, while resembling that of an atoll, has a very different mode of formation. While there is no atoll directly on the Equator, the closest atoll to the Equator is Aranuka of Kiribati, with its southern tip just 12 km north of the Equator.

Largest atolls by total area (lagoon plus reef and dry land) [11]
Area (km2)
Great Chagos Bank 6°10′S72°00′E / 6.17°S 72.00°E / -6.17; 72.00
Indian Ocean
Land area 4.5 km2
Reed Bank 11°27′N116°54′E / 11.45°N 116.90°E / 11.45; 116.90
Spratly Islands
Submerged, at shallowest 9 m
Macclesfield Bank 16°00′N114°30′E / 16.00°N 114.50°E / 16.00; 114.50
South China Sea
Submerged, at shallowest 9.2 m
North Bank (Ritchie Bank, north of
Saya de Malha Bank)
9°04′S60°12′E / 9.07°S 60.20°E / -9.07; 60.20
North of Saya de Malha Bank
Submerged, at shallowest <10 m
Rosalind Bank 16°26′N80°31′W / 16.43°N 80.52°W / 16.43; -80.52
Submerged, at shallowest 7.3 m
Thiladhunmathi (Boduthiladhunmathi) 6°44′N73°02′E / 6.73°N 73.04°E / 6.73; 73.04
Land area 51 km2
Chesterfield Islands 19°21′S158°40′E / 19.35°S 158.66°E / -19.35; 158.66
New Caledonia
Land area <10 km2
Huvadhu Atoll 0°30′N73°18′E / 0.50°N 73.30°E / 0.50; 73.30
Land area 38.5 km2
Truk Lagoon 7°25′N151°47′E / 7.42°N 151.78°E / 7.42; 151.78
Chuuk, FSM
Sabalana Islands 6°45′S118°50′E / 6.75°S 118.83°E / -6.75; 118.83
Nukuoro atoll 3°51′N154°56′E / 3.85°N 154.94°E / 3.85; 154.94
Pohnpei, FSM
Land area 1.7 km2 in 40 islets
Lihou Reef 17°25′S151°40′E / 17.42°S 151.67°E / -17.42; 151.67
Coral Sea
Land area 1 km2
Bassas de Pedro 13°05′N72°25′E / 13.08°N 72.42°E / 13.08; 72.42
Lakshadweep, India
Submerged, at shallowest 16.4 m
Ardasier Bank 7°43′N114°15′E / 7.71°N 114.25°E / 7.71; 114.25
Spratly Islands
Cay on the south side?
Kwajalein 9°11′N167°28′E / 9.19°N 167.47°E / 9.19; 167.47
Marshall Islands
Land area 16.4 km2
Diamond Islets Bank 17°25′S150°58′E / 17.42°S 150.96°E / -17.42; 150.96
Coral Sea
Land area <1 km2
Namonuito Atoll 8°40′N150°00′E / 8.67°N 150.00°E / 8.67; 150.00
Chuuk, FSM
Land area 4.4 km2
Ari Atoll 3°52′N72°50′E / 3.86°N 72.83°E / 3.86; 72.83
Land area 69 km2
Maro Reef 25°25′N170°35′W / 25.42°N 170.59°W / 25.42; -170.59
Northwestern Hawaiian Islands
Rangiroa 15°08′S147°39′W / 15.13°S 147.65°W / -15.13; -147.65
Tuamotu Islands
Land area 79 km2
Kolhumadulhu Atoll 2°22′N73°07′E / 2.37°N 73.12°E / 2.37; 73.12
Land area 79 km2
Kaafu Atoll (North Malé Atoll) 4°25′N73°30′E / 4.42°N 73.50°E / 4.42; 73.50
Land area 69 km2
Ontong Java 5°16′S159°21′E / 5.27°S 159.35°E / -5.27; 159.35
Solomon Islands
Land area 12 km2

In most cases, the land area of an atoll is very small in comparison to the total area. Atoll islands are low lying, with their elevations less than 5 meters (9). Measured by total area, Lifou (1146 km2) is the largest raised coral atoll of the world, followed by Rennell Island (660 km2). [13] More sources, however, list Kiritimati as the largest atoll in the world in terms of land area. It is also a raised coral atoll (321.37 km2 land area; according to other sources even 575 km2), 160 km2 main lagoon, 168 km2 other lagoons (according to other sources 319 km2 total lagoon size).

The remains of an ancient atoll as a hill in a limestone area is called a reef knoll. The second largest atoll by dry land area is Aldabra, with 155 km2. The largest atoll in terms of island numbers is Huvadhu Atoll in the south of the Maldives, with 255 islands.

Map from Charles Darwin's 1842 The Structure and Distribution of Coral Reefs showing the world's major groups of atolls and coral reefs. On the structure and distribution of coral reefs BHL40453231.jpg
Map from Charles Darwin’s 1842 The Structure and Distribution of Coral Reefs showing the world’s major groups of atolls and coral reefs.


Aerial view of Bora Bora Bora Bora (16542797633).jpg
Aerial view of Bora Bora
Tarawa Atoll South Tarawa from the air.jpg
Tarawa Atoll
Bikini Atoll Bikini Atoll.png
Bikini Atoll

In 1842, Charles Darwin explained the creation of coral atolls in the southern Pacific Ocean based upon observations made during a five-year voyage aboard HMS Beagle from 1831 to 1836. Accepted as basically correct, his explanation suggested that several tropical island types: from high volcanic island, through barrier reef island, to atoll, represented a sequence of gradual subsidence of what started as an oceanic volcano. He reasoned that a fringing coral reef surrounding a volcanic island in the tropical sea will grow upward as the island subsides (sinks), becoming an "almost atoll", or barrier reef island, as typified by an island such as Aitutaki in the Cook Islands, and Bora Bora and others in the Society Islands. The fringing reef becomes a barrier reef for the reason that the outer part of the reef maintains itself near sea level through biotic growth, while the inner part of the reef falls behind, becoming a lagoon because conditions are less favorable for the coral and calcareous algae responsible for most reef growth. In time, subsidence carries the old volcano below the ocean surface and the barrier reef remains. At this point, the island has become an atoll.

Atolls are the product of the growth of tropical marine organisms, and so these islands are found only in warm tropical waters. Volcanic islands located beyond the warm water temperature requirements of hermatypic (reef-building) organisms become seamounts as they subside, and are eroded away at the surface. An island that is located where the ocean water temperatures are just sufficiently warm for upward reef growth to keep pace with the rate of subsidence is said to be at the Darwin Point. Islands in colder, more polar regions evolve toward seamounts or guyots; warmer, more equatorial islands evolve toward atolls, for example Kure Atoll.

Reginald Aldworth Daly offered a somewhat different explanation for atoll formation: islands worn away by erosion, by ocean waves and streams, during the last glacial stand of the sea of some 900 feet (270 m) below present sea level developed as coral islands (atolls), or barrier reefs on a platform surrounding a volcanic island not completely worn away, as sea levels gradually rose from melting of the glaciers. Discovery of the great depth of the volcanic remnant beneath many atolls, such as at Midway Atoll, favors the Darwin explanation. But fluctuating sea level has also had considerable influence on atolls and other reefs.

Coral atolls are important as places where dolomitization of calcite occurs. At certain depths water is undersaturated in calcium carbonate but saturated in dolomite. Convection created by tides and sea currents enhances this change. Hydrothermal currents created by volcanoes under the atoll may also play an important role.

Investigation by the Royal Society of London into the formation of coral reefs

In 1896, 1897 and 1898, the Royal Society of London carried out drilling on Funafuti atoll in Tuvalu for the purpose of investigating the formation of coral reefs. They wanted to determine whether traces of shallow water organisms could be found at depth in the coral of Pacific atolls. This investigation followed the work on the structure and distribution of coral reefs conducted by Charles Darwin in the Pacific.

The first expedition in 1896 was led by Professor William Johnson Sollas of the University of Oxford. Geologists included Walter George Woolnough and Edgeworth David of the University of Sydney. Professor Edgeworth David led the expedition in 1897. [14] The third expedition in 1898 was led by Alfred Edmund Finckh. [15] [16] [17]

Aerial overview of the Wake Island atoll, part of the Pacific Remote Islands Marine National Monument Wake Island air.JPG
Aerial overview of the Wake Island atoll, part of the Pacific Remote Islands Marine National Monument

United States national monuments

On January 6, 2009, U.S. President George W. Bush announced the creation of the Pacific Remote Islands Marine National Monument, covering several islands and atolls under U.S. jurisdiction. [18] [19] (Number 1, page 14)

See also

Related Research Articles

Coral Sea Islands

The Coral Sea Islands Territory is an external territory of Australia which comprises a group of small and mostly uninhabited tropical islands and reefs in the Coral Sea, northeast of Queensland, Australia. The only inhabited island is Willis Island. The territory covers 780,000 km2 (301,160 sq mi), most of which is ocean, extending east and south from the outer edge of the Great Barrier Reef and includes Heralds Beacon Island, Osprey Reef, the Willis Group and fifteen other reef/island groups. Cato Island is the highest point in the Territory.

Island Any piece of sub-continental land that is surrounded by water

An island or isle is any piece of subcontinental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, skerries, cays or keys. An island in a river or a lake island may be called an eyot or ait, and a small island off the coast may be called a holm. Sedimentary islands in the Ganges delta are called chars. A grouping of geographically or geologically related islands, such as the Philippines, is referred to as an archipelago.

Geography of Tuvalu

The Western Pacific nation of Tuvalu, formerly known as the Ellice Islands. It is situated 4,000 kilometers (2,500 mi) northeast of Australia and is approximately halfway between Hawaii and Australia. It lies east-northeast of the Santa Cruz Islands, southeast of Nauru, south of Kiribati, west of Tokelau, northwest of Samoa and Wallis and Futuna and north of Fiji. It is a very small island country of and is 26 km2 (10 sq mi). Due to the spread out islands it has the 38th largest Exclusive Economic Zone of 749,790 km2 (289,500 sq mi).

Coral reef Outcrop of rock in the sea formed by the growth and deposit of stony coral skeletons

A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.

Guyot An isolated, flat-topped underwater volcano mountain

In marine geology, a guyot, also known as a tablemount, is an isolated underwater volcanic mountain (seamount) with a flat top more than 200 m (660 ft) below the surface of the sea. The diameters of these flat summits can exceed 10 km (6.2 mi). Guyots are most commonly found in the Pacific Ocean, but they have been identified in all the oceans except the Arctic Ocean.

Reef A shoal of rock, sand, coral or similar material, lying beneath the surface of water

A reef is a shoal of rock, sand, coral or similar material, lying beneath the surface of water. Many reefs result from natural, abiotic processes—deposition of sand, wave erosion planing down rock outcrops, etc.—but the best known reefs are the coral reefs of tropical waters developed through biotic processes dominated by corals and coralline algae.

Kure Atoll Atoll of the Hawaiian Islands

Kure Atoll or Ocean Island is an atoll in the Pacific Ocean 48 nautical miles west-northwest of Midway Atoll in the Northwestern Hawaiian Islands at 28°25′N178°20′W. The only land of significant size is called Green Island and is a habitat for hundreds of thousands of seabirds. A short, unused and unmaintained runway and a portion of one building, both from a former United States Coast Guard LORAN station, are located on the island. Politically, it is part of Hawaii, although separated from the rest of the state by Midway, which is a separate unorganized territory. Green Island, in addition to being the nesting grounds of tens of thousands of seabirds, has recorded several vagrant terrestrial birds including snow bunting, eyebrowed thrush, brambling, olive-backed pipit, black kite, Steller's sea eagle and Chinese sparrowhawk. It is currently managed as a Wildlife Bird Sanctuary by the State of Hawaii's Department of Land and Natural Resource--Division of Forestry and Wildlife as one of the co-trustees of Papahānaumokuākea Marine National Monument with support from Kure Atoll Conservancy.

Northwestern Hawaiian Islands Small islands and atolls in Hawaii

The Northwestern Hawaiian Islands or Leeward Islands are the small islands and atolls in the Hawaiian island chain located northwest of the islands of Kauai and Niihau. Politically, they are all part of Honolulu County in the U.S. state of Hawaii, except Midway Atoll, which is a territory distinct from Hawaii and grouped as one of the United States Minor Outlying Islands. The United States Census Bureau defines this area, except Midway, as Census Tract 114.98 of Honolulu County. Its total land area is 3.1075 square miles (8.048 km2). All the islands except Nihoa are north of the Tropic of Cancer, making them the only islands in Hawaii that lie outside the tropics.

Evolution of Hawaiian volcanoes Processes of growth and erosion of the volcanoes of the Hawaiian islands

The fifteen volcanoes that make up the eight principal islands of Hawaii are the youngest in a chain of more than 129 volcanoes that stretch 5,800 kilometres (3,600 mi) across the North Pacific Ocean, called the Hawaiian–Emperor seamount chain. Hawaiʻi's volcanoes rise an average of 4,600 metres (15,000 ft) to reach sea level from their base. The largest, Mauna Loa, is 4,169 metres (13,678 ft) high. As shield volcanoes, they are built by accumulated lava flows, growing a few meters or feet at a time to form a broad and gently sloping shape.

Coral island

A coral island is a type of island formed from coral detritus and associated organic material. They occur in tropical and sub-tropical areas, typically as part of coral reefs which have grown to cover a far larger area under the sea.

Funafuti Capital of Tuvalu

Funafuti is an atoll and the capital of the island nation of Tuvalu. It has a population of 6,320 people, making it the country's most populated atoll, with 60.15 percent of Tuvalu's population. It is a narrow sweep of land between 20 and 400 metres wide, encircling a large lagoon 18 km long and 14 km wide. The average depth in the Funafuti lagoon is about 20 fathoms. With a surface of 275 square kilometres (106.2 sq mi), it is by far the largest lagoon in Tuvalu. The land area of the 33 islets aggregates to 2.4 square kilometres (0.9 sq mi), less than one percent of the total area of the atoll. Cargo ships can enter Funafuti's lagoon and dock at the port facilities on Fongafale.

<i>The Structure and Distribution of Coral Reefs</i>

The Structure and Distribution of Coral Reefs, Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836, was published in 1842 as Charles Darwin's first monograph, and set out his theory of the formation of coral reefs and atolls. He conceived of the idea during the voyage of the Beagle while still in South America, before he had seen a coral island, and wrote it out as HMS Beagle crossed the Pacific Ocean, completing his draft by November 1835. At the time there was great scientific interest in the way that coral reefs formed, and Captain Robert FitzRoy's orders from the Admiralty included the investigation of an atoll as an important scientific aim of the voyage. FitzRoy chose to survey the Keeling Islands in the Indian Ocean. The results supported Darwin's theory that the various types of coral reefs and atolls could be explained by uplift and subsidence of vast areas of the Earth's crust under the oceans.

Nintoku Seamount A flat topped seamount in the Hawaiian-Emperor seamount chain

Nintoku Seamount or Nintoku Guyot is a seamount and guyot in the Hawaiian-Emperor seamount chain. It is a large, irregularly shaped volcano that last erupted 66 million years ago. Three lava flows have been sampled at Nintoku Seamount; the flows are almost all alkalic (subaerial) lava. It is 56.2 million years old.

Mid-Pacific Mountains An underwater mountain range from the southern tier of the Japan Trench to the Hawaiian Islands

The Mid-Pacific Mountains (MPM) is a large oceanic plateau located in the central North Pacific Ocean or south of the Hawaiian–Emperor seamount chain. Of volcanic origin and Mesozoic in age, it is located on the oldest part of the Pacific Plate and rises up to 2 km (1.2 mi) above the surrounding ocean floor and is covered with several layers of thick sedimentary sequences that differ from those of other plateaux in the North Pacific. About 50 seamounts are distributed over the MPM. Some of the highest points in the range are above sea level which include Wake Island and Marcus Island.

Wōdejebato guyot in the Marshall Islands northwest of the smaller Pikinni Atoll

Wōdejebato is a Cretaceous guyot or tablemount in the northern Marshall Islands, Pacific Ocean. Wōdejebato is probably a shield volcano and is connected through a submarine ridge to the smaller Pikinni Atoll 74 kilometres (46 mi) southeast of the guyot; unlike Wōdejebato, Pikinni rises above sea level. The seamount rises for 4,420 metres (14,500 ft) to 1,335 metres (4,380 ft) depth and is formed by basaltic rocks. The name Wōdejebato refers to a sea god of Pikinni.

Limalok Cretaceous-Paleocene guyot in the Marshall Islands

Limalok is a Cretaceous-Paleocene guyot/tablemount in the southeastern Marshall Islands, one of a number of seamounts in the Pacific Ocean. It was probably formed by a volcanic hotspot in present-day French Polynesia. Limalok lies southeast of Mili Atoll and Knox Atoll, which rise above sea level, and is joined to each of them through a volcanic ridge. It is located at a depth of 1,255 metres (4,117 ft) and has a summit platform with an area of 636 square kilometres (246 sq mi).

Lo-En Albian–Campanian guyot in the Marshall Islands in the Pacific Ocean

Lo-En or Hess is an Albian–Campanian guyot in the Marshall Islands. One among a number of seamounts in the Pacific Ocean, it was probably formed by a hotspot in what is present-day French Polynesia. Lo-En lies southeast of Eniwetok which rises above sea level, and Lo-En is almost connected to it through a ridge.

Allison Guyot Seamount in the Pacific Ocean

Allison Guyot is a tablemount (guyot) in the underwater Mid-Pacific Mountains of the Pacific Ocean. It is a trapezoidal flat mountain rising 1,500 metres above the seafloor to a depth of less than 1,500 m, with a summit platform 35 by 70 kilometres wide. The Mid-Pacific Mountains lie west of Hawaii and northeast of the Marshall Islands, but at the time of their formation were located in the Southern Hemisphere.

Darwin Guyot

Darwin Guyot is a volcanic underwater mountain top, or guyot, in the Mid-Pacific Mountains between the Marshall Islands and Hawaii. Named after Charles Darwin, it rose above sea level more than 118 million years ago during the early Cretaceous period to become an atoll, developed rudist reefs, and then drowned, perhaps as a consequence of sea level rise. The flat top of Darwin Guyot now rests 1,266 metres (4,154 ft) below sea level.


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